ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

27 of 42

www.diodes.com

February 2012

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Inductor Selection and frequency Control

The selection of the inductor coil, L1, requires knowledge of the switching frequency and current ripple, and also depends on

the duty cycle to some extent. In the hysteretic converter, the frequency depends upon the input and output voltages and

the switching thresholds of the current monitor. The peak-to-peak coil current is adjusted by the ZXLD1371 to control the

frequency to a fixed value. This is done by controlling the switching thresholds within particular limits. This effectively much

reduces the overall frequency range for a given input voltage range. Where the input voltage range is not excessive, the

frequency is regulated to approximately 390kHz. This is helpful in terms of EMC and other system requirements. Figure 7

shows practical results of switching frequency driving 8 LEDs at 350mA.

350mA LED Current and Various Inductor Values

For larger input voltage variation, or when the choice of coil inductance is not optimum, the switching frequency may depart

from the regulated value, but the regulation of LED current remains successful. If desired, the frequency can to some extent

be increased by using a smaller inductor, or decreased using a larger inductor.

The web Calculator will evaluate the

frequency across the input voltage range and the effect of this upon power efficiency and junction temperatures.

Determination of the input voltage range for which the frequency is regulated may be required. This calculation is very

involved, and is not given here. However the performance in this respect can be evaluated within the web Calculator for the

chosen inductance.

The inductance is given as follows in terms of peak-to-peak ripple current in the coil,

L1 =

∆I

L

for Buck

L1 =

∆I

L

for Boost

L1 =

∆I

L

for Buck-boost

Equation 18

estimated.

D

f

Equation 19

As the regulated frequency is known, and we have already found D from Equation 7 or the approximation Equation 7b, this

6

57

8

10

911

12

13

14

15

16

17

18

19

20

V

(V)

IN

T = 25 C, V

= V

8 LEDs, R = 150m ,

R9 = 120k , R10 = 36k ,

C

= 100µF

AAUX

IN

S

IN

°

Ω

ΩΩ

L = 33µH

0

50

100

150

200

250

300

350

400

450

500

L = 100µH

L = 68µH